It is an excellent review of the goals and workings of the law, and it presents excellent ideas for balancing what likely will continue to be ''an unavoidable clash among society's fundamental values.'' Most important, it does this in a way that is dispassionate, yet earnest and clear-headed.

When the United States in 1973 adopted what still is the most profound environmental-protection law in the world, it took on a challenge of Biblical proportions: how to distinguish from among millions of species those whose numbers are declining because of the acts of mankind, and how to save them from extinction.

Today, the Endangered Species Act is the subject of much controversy. The list of endangered and threatened species is longer than ever (1,512), the number of recovered species is pitifully small, the economic and social costs of saving what many feel to be insignificant plants and animals have been high, and the political outcry has been loud and shrill.

Mann and Plummer are neither scientists nor partisans, and this is to the reader's benefit.

In plain language (and with plenty of context and perspective), they take us through species definition, habitat, theories behind the cause and rate of extinction, and other biological details that could have become a muddle to the nonspecialist. With even-handedness, they present the assertions and difficulties of those in government and the private sector wrestling with enforcement.

What could have been a dry tract is made compelling by the stories of recent struggles over species preservation versus development: the Karner Blue butterfly in Albany, N.Y.; the whooping crane in Nebraska; the snail darter in Tennessee; the black-capped vireo in Austin, Texas.

One of their key points is that most species declines result from thousands of individual actions. Likewise, they assert, restoration will require ''hundreds of individual actions, few of them noteworthy regarded in isolation.''

Extremists at both ends of the political debate are wrong, the authors credibly maintain. Those who deny that the rate of extinction is greater than what should be normal are wrong. So too are those who cling to the Endangered Species Act as akin to scripture in importance and untouchability.

The problem with the law, the authors contend, is that it relies so exclusively on science that it leaves little room for other points of view in the policy discussion. ''Relying solely on experts to determine biodiversity policy is as absurd as using public referenda to decide which species are endangered. Science needs to resume its proper place: providing support for policymakers. But -- and this is vital -- policymakers cannot be required to follow the dictates of experts, whether they are economists or ecologists,'' they write.

They are not arguing that developers should be able to bulldoze the habitat of endangered plants and animals. And they agree that biological diversity is essential. Saving species, they say, will require a greater effort, including some sacrifices to special interests and more support from all Americans through the public treasury.

But most important is that there be an informed and cooperative effort all around, and that this effort be politically inclusive. ''[T]hrusting biodiversity into the rough-and-tumble world of politics will elevate it alongside such basic values as health, defense, and education, giving it legitimate claim to a greater share of public resources,'' they write. ''No longer able to foist the hard choices onto blue-ribbon panels of scientists, political decisionmakers will be forced to educate themselves about biodiversity issues.''